Exhaust gas purifying apparatus for internal combustion engine

ABSTRACT

An exhaust pipe is provided with an oxidation catalyst, a SCR catalyst (ammonia selective reduction catalyst), and an ammonia slip catalyst. In the exhaust pipe, the urea water adding valve is provided between the oxidation catalyst and the SCR catalyst. A NOx sensor which detects the NOx quantity in an exhaust gas is provided downstream of the SCR catalyst. An ECU controls the urea water adding valve to add the urea water to the exhaust gas. While the urea water is added to the exhaust gas, the ECU successively obtains a NOx sensor output, and computes an adding quantity command value in which the NOx sensor output becomes minimum.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2007-290828filed on Nov. 8, 2007, the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to an exhaust gas purifying apparatus foran internal combustion engine, which is preferably applied to an exhaustgas purifying system of Selective Catalytic Reduction (SCR) type usingammonia reducing agent such as urea aqueous solution (urea water).

BACKGROUND OF THE INVENTION

A urea Selective Catalytic Reduction (SCR) system has been developed asan exhaust gas purifying apparatus which purifies nitrogen oxide (NOx)in an exhaust gas exhausted from an internal combustion engine,especially from a diesel engine. The urea SCR system has followingconfiguration:

That is, in the urea SCR system, a selective catalytic reduction typeNOx reduction catalyst (SCR catalyst) is provided in an exhaust gas pipeconnected to an engine body, and a urea water adding valve (UWA valve)is provided upstream of the SCR catalyst in order to add a urea waterinto the exhaust pipe. The exhaust gas and the urea water are suppliedto the NOx reduction catalyst so that the exhaust gas is purified by areductive reaction of NOx on the NOx reduction catalyst. In resolvingNOx, the urea water is hydrolyzed by exhaust gas heat to generateammonia (NH₃), and the NOx is selectively reduced by ammonia on the NOxreduction catalyst, whereby the exhaust gas is purified.

A NOx sensor is provided downstream of the NOx reduction catalyst sothat NOx concentration is detected. Based on an output of the NOxsensor, a NOx purifying ratio is computed. In the exhaust gas purifyingapparatus shown in JP-2003-314256A, NOx sensors are respectivelyprovided upstream and downstream of the NOx reduction catalyst. Based oneach output of the NOx sensors, the NOx purifying ratio is computed.Besides, while the engine is stably running, a supply condition of thereducing agent is switched to compute a difference in the NOx purifyingratio between a case where the reducing agent is supplied and a casewhere no reducing agent is supplied. Based on the difference in the NOxpurifying ratio, an ammonia adsorbed quantity and the reducing agentadded quantity are computed.

Generally, the NOx sensor includes a sensor element comprised of a solidelectrolyte and a pair of electrodes, and senses ammonia (NH₃) as wellas NOx. When ammonia excessive for the NOx reduction catalyst isdischarged to downstream of the catalyst (when ammonia slip isgenerated), the output of the NOx sensor is varied according to adischarged quantity of ammonia. In such a case, there is a possibilitythat the NOx purifying ratio is erroneously computed based on the NOxsensor output. If the accuracy of the NOx purifying ratio isdeteriorated, the accuracy of the ammonia adsorbed quantity and thereducing agent added quantity is also deteriorated, which may cause adecrease of the NOx purifying ratio and an increase of the ammonia slipquantity.

SUMMARY OF THE INVENTION

The present invention is made in view of the above matters, and it is anobject of the present invention to provide an exhaust gas purifyingapparatus for an internal combustion engine, which can correctly detectNOx quantity downstream of the NOx reduction catalyst so that the NOxpurifying ratio can be appropriately computed.

According to the present invention, an exhaust gas purifying apparatusfor an internal combustion engine includes a NOx reduction catalyst (forexample, ammonia selective reduction catalyst) provided in an exhaustgas pipe. Reducing agent (for example, ammonia reducing agent such asurea aqueous solution) is added to the exhaust gas, and NOx purificationis performed in the NOx reduction catalyst. NOx quantity downstream ofthe NOx reduction catalyst is detected by a NOx sensor. A NOx purifyingratio is computed based on the detected NOx quantity.

An adding quantity control means controls an adding quantity of thereducing agent by the reducing agent adding means. While the reducingagent is added to the exhaust gas, a NOx sensor output is successivelyobtained and an adding quantity command value in which the NOx sensoroutput becomes minimum is computed.

The NOx sensor detects the reducing agent as well as NOx in the exhaustgas, which becomes surplus for NOx reduction catalyst. When the ureawater adding quantity is increased, NOx concentration downstream of NOxreduction catalyst is gradually decreased. Furthermore, when thereducing agent is increased, the ammonia concentration (ammonia slipquantity) is increased. In this case, as shown FIG. 4C, the NOx sensoroutput line is downwardly convex with respect to the reducing agent(urea water) adding quantity. When the NOx sensor output is minimum, theNOx concentration and the ammonia concentration downstream of the NOxreduction catalyst are low and the NOx purifying ratio is maximum.

According to the present invention, the adding quantity of the reducingagent is variously changed, the NOx sensor output is obtained withrespect to each adding quantity of the reducing agent, and an addingquantity command value is computed according to the adding quantity ofthe reducing agent in which the NOx sensor output becomes minimum.

Thereby, the surplus quantity of the reducing agent (ammonia slipquantity) is reduced and the NOx purifying ratio becomes maximum. As theresult, the NOx quantity downstream of the NOx reduction catalyst iscorrectly detected, so that the NOx purifying ratio can be properlycomputed.

According to another aspect of the invention, the adding quantity of thereducing agent may be varied at least into increasing side or decreasingside relative to the adding quantity command value as a reference.According to this configuration, it can be grasped properly whether theadding quantity command value is optimum and whether the adding quantitymakes the NOx sensor output minimum.

According to another aspect of the present invention, at least threesteps of reducing agent additions are performed while the addingquantity of the reducing agent is varied by a predetermined variationwidth. Further, the reducing agent addition is performed again while thevariation width is made small in a case that the sensor output becomesminimum with respect to a medium quantity of reducing agent among atleast three steps of reducing agent additions

That is, there is a possibility that the most appropriate addingquantity command value may exists between the maximum adding quantityand the minimum adding quantity, in which the NOx sensor output becomesminimum. In such a case, the reducing agent addition is performed againwhile the variation width is made small, whereby the optimum value ofthe adding quantity command value can be accurately obtained.

According to another aspect of the invention, based on a differencebetween the NOx sensor output before changing the reducing agent addingquantity and the NOx sensor output after changing the reducing agentadding quantity, it can be estimated whether the reducing agent addingquantity in which the NOx sensor output is minimum is in an increasingside or a decreasing side. Based on this estimated result, the reducingagent adding quantity may be increased or decreased. For example, whenthe NOx sensor output is increased due to the variation in the reducingagent adding quantity, an increase/decrease direction of the reducingagent adding quantity is reversed. Alternatively, when the NOx sensoroutput is decreased due to the variation in the reducing agent addingquantity, the reducing agent adding quantity is varied in the sameincrease/decrease direction.

According to this configuration, the reducing agent adding quantity isvaried only in a direction where the minimum sensor output exists. Thus,the increase/decrease process of the reducing agent adding quantity canbe simplified.

According to another aspect of the invention, a variation width of thereducing agent adding quantity may be varied based on the output valueof the NOx sensor. For example, as the NOx sensor output increases, thevariation width of the reducing agent adding quantity increases. Whenthe NOx sensor output is relatively large, the variation ratio of theNOx sensor output is also relatively large with respect to a variationin the reducing agent adding quantity. When the NOx sensor output isrelatively small, the variation ratio of the NOx sensor output is alsorelatively small. Thus, it is desirable to set the variation width ofthe reducing agent adding quantity based on the output value of the NOxsensor.

According to another aspect of the invention, it is desirable that theadding quantity command value is not newly computed when a differencebetween a minimum value of the sensor output and a maximum value of thesensor output, which are obtained due to a variation in adding quantityof the reducing agent, is within a specified value. That is, at vicinitywhere the NOx sensor output is minimum, the NOx sensor output hardlyvaries even if the reducing agent adding quantity is varied. Hence,unnecessary computation (update) of the adding quantity command valuecan be avoided.

According to another aspect of the invention, a period during which thereducing agent adding quantity is decreased is longer than a periodduring which the reducing agent adding quantity is increased. That is,comparing a case that the urea water adding quantity is increased with acase that the urea water adding quantity is decreased, a response speedof the NOx sensor differs. The response speed is slow in the lattercase. This is because the reducing agent consuming speed in the NOxreduction catalyst is slower than the reducing agent adsorbing speed(ammonia adsorbing speed) in the NOx reduction catalyst. According tothe above configuration, the NOx sensor output can be appropriatelyobtained even in both cases where the reducing agent adding quantity isincreased or decreased.

The adding quantity command value in which the NOx sensor output isminimum does not vary successively. When the engine driving condition isstable, the adding quantity command value is constant value. Thus, it isdesirable to store the adding quantity command value in a back up memoryas a learning value and to update the learning value as needed. Thereby,since the adding quantity command value may be computed in a minimumfrequency, a computation load for computing the adding quantity commandvalue can be reduced. For example, every when an ECU is energized, theadding quantity command value may be computed only once.

The characteristic of NOx sensor output with respect to the reducingagent adding quantity varies according to the engine driving condition.Thus, it is desirable that the adding quantity command value is storedin the memory along with a driving condition of the internal combustionengine at a time of controlling the adding quantity of the reducingagent. Thereby, even if the driving condition of the engine is varied,an appropriate adding quantity command value can be established.

According to another aspect of the invention, an exhaust gas purifyingapparatus includes an oxidation catalyst (for example, ammonia slipcatalyst) which is arranged downstream of the NOx reduction catalyst forpurifying the reducing agent, and a determination means for determiningwhether the oxidation catalyst is active or not. When the determinationmeans determines the oxidation catalyst is inactive, the command valuecomputing means performs a computation of the adding quantity commandvalue.

That is, when the oxidation catalyst downstream of the NOx reductioncatalyst is inactive, if the reducing agent is discharged downstream ofthe NOx reduction catalyst, the reducing agent may not be appropriatelypurified. According to the above configuration, when the oxidationcatalyst is inactive, the adding quantity command value is computed sothat it can be restricted that the reducing agent is dischargeddownstream of the NOx reduction catalyst and the reducing agent isdischarged into the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following description made with referenceto the accompanying drawings, in which like parts are designated by likereference numbers and in which:

FIG. 1 is a construction view schematically showing an engine controlsystem in an embodiment of the invention;

FIGS. 2A-2C are time charts for explaining valve open command pulses;

FIG. 3 is a cross sectional view showing a sensor element of a NOxsensor;

FIGS. 4A-4C are graphs respectively showing NOx concentration, NH₃concentration, and NOx sensor output downstream of a catalyst withrespect to a urea water adding quantity;

FIG. 5 is a flowchart showing a urea water adding quantity controlprocess;

FIG. 6 is a flowchart showing an increase/decrease process of a ureawater adding quantity;

FIG. 7 is a graph showing a characteristic of a NOx sensor output;

FIGS. 8A and 8B are time charts showing a transition of the NOx sensoroutput in a case of decreasing the urea water adding quantity;

FIGS. 9A and 9B are graphs schematically showing the NOx sensor output;

FIGS. 10A and 10B are graphs schematically showing the NOx sensoroutput; and

FIGS. 11A and 11B are graphs schematically showing the NOx sensoroutput.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereafter, an embodiment of the present invention is described. In thisembodiment, a multi-cylinder diesel engine is controlled. An electroniccontrol unit (ECU) performs a various kind of controls to the engine.The diesel engine has a common-rail fuel injection system and a urea SCRsystem, Referring to FIG. 1, the system is schematically explained,hereinafter.

The engine 10 includes an engine body 11 which has a piston 12, anintake valve 13, and an exhaust valve 14. A reciprocative movement ofthe piston 12 rotates a crankshaft 15. A fuel injector 16 is provided ona cylinder head with respect to each cylinder. The fuel injector 16injects fuel into a combustion chamber 17 directly, which is combustedin the combustion chamber 17.

The crankshaft 15 is provided with a crank angle sensor 18 which detectsa rotation of the crankshaft 15. A cylinder block is provided with acoolant temperature sensor 19 which detects coolant temperature.

A fuel supply system will be briefly described hereinafter. The fuelsupply system is provided with a high-pressure pump and a common rail.The high-pressure pump pumps up the fuel in the fuel tank and suppliesthe fuel to the common rail. The high pressure fuel of 10-200 MPa isstored in the common rail and is supplied to the fuel injector 16 ofeach cylinder. The fuel pressure in the common rail is suitably adjustedaccording to the engine driving condition.

An intake pipe (including a manifold portion) 21 is connected to anintake port of the engine 11. An exhaust pipe (including a manifoldportion) 22 is connected to an exhaust port of the engine 11. The intakepipe 21 is provided with a throttle actuator 23 which includes anelectric drive throttle valve. The intake pipe 21 and the exhaust pipe22 are connected with each other through an EGR pipe 24. The EGR pipe 24is provided with an EGR valve 25 and an EGR cooler 26. An air cleaner 27is provided at a most upstream portion of the intake pipe 21.

This fuel supply system is provided with a turbocharger 30. Theturbocharger 30 is provided with an intake compressor 31 arranged in theintake pipe 21 and an exhaust turbine 32 arranged in the exhaust pipe22. The exhaust turbine 32 is rotated by the exhaust gas flowing throughthe exhaust pipe 22. This rotation is transmitted to the intakecompressor through a shaft 33. The intake compressor 31 compresses theintake air flowing through the intake pipe 21. The compressed air iscooled by an intercooler 34 and supplied to downstream of the intakepipe 21.

The intake pipe 21 is provided with a various kind of sensors, such asan air-flow meter, an intake air pressure sensor, an intake airtemperature sensor, and the like.

The exhaust gas purifying system will be described hereinafter. Theexhaust pipe 22 is provided with an oxidation catalyst 41, a SCRcatalyst (ammonia selective reduction catalyst) 42, and an ammonia slipcatalyst 43. The SCR catalyst 42 corresponds to the NOx reductioncatalyst. Between the oxidation catalyst 41 and the SCR catalyst 42, aurea water adding valve (UWA valve) 44 is provided in the exhaust pipe22 in order to supply the urea water as the reducing agent into theexhaust pipe 22. The UWA valve 44 has substantially the same structureas a well-known fuel injector, and injects the urea water from itsinjection port on receiving an injection command signal. A urea watertank (not shown) stores the urea water therein. The urea water issuccessively supplied to the UWA valve 44 by a urea water supply pump(not shown) while the engine is running.

When the urea water is injected into the exhaust pipe 22, the exhaustgas and the urea water flow into the SCR catalyst 42 in which thereductive reaction of NOx is performed to purify the exhaust gas.

Specifically, the injected urea water is hydrolyzed to generate ammonia(NH₃) as described in following chemical equation.

(NH₂)₂CO+H₂O→2NH₃+CO₂  (1)

When the exhaust gas flows through the SCR catalyst 42, NOx in theexhaust gas is selectively reduced as described in following chemicalequations.

4NO+4NH₃+O₂→4N₂+6H₂O  (2)

6NO₂+8NH₃→7N₂+12H₂O  (3)

NO+NO₂+2NH₃→2N₂+3H₂O  (4)

Non-reacted ammonia is discharged with the exhaust gas into thedownstream. Non-reacted ammonia is removed by the ammonia slip catalyst43 arranged downstream of the SCR catalyst 42.

An oxygen concentration sensor 45 and an exhaust gas temperature sensor46 are provided in the exhaust pipe 22 between an oxidation catalyst 45and the SCR catalyst 46 in order to detect the oxygen concentration inthe exhaust gas and the exhaust gas temperature. A NOx sensor 47detecting NOx concentration in the exhaust gas is arranged downstream ofthe SCR catalyst 42. Based on an output of the NOx sensor, the NOxpurifying ratio of the SCR catalyst 42 is computed.

The exhaust pipe 22 is provided with a diesel particulate filter (DPF:not shown) capturing particulate matters (PM) in the exhaust gas.

The ECU 50 includes a microcomputer comprised of a CPU, a ROM, a RAM andthe like. The ECU 50 receives detected signals from the above sensors, arail pressure sensor detecting fuel pressure in the common rail, anaccelerator sensor detecting accelerator operated quantity, and thelike. The ECU 50 performs a fuel injection control, a fuel pressurecontrol (rail pressure control) and the like based on the engine speed,the accelerator operated quantity and the like. The fuel injectionoperation of the fuel injector 16 and the fuel pumping operation of thehigh-pressure pump are controlled. Furthermore, the ECU 50 controls thethrottle actuator 23 and the EGR valve 25 based on the current enginedriving condition.

The ECU 50 has an EEPROM 51 as a backup memory. The EEPROM 51 stores avarious leaning value and diagnosis data. A standby RAM can be used asthe backup memory in stead of the EEPROM.

The ECU 50 computes the NOx quantity downstream of the SCR catalyst 42and the NOx purifying ratio based on the output of the NOx sensor 47.Further, the ECU 50 controls a urea water adding quantity based on theNOx purifying ratio. The NOx purifying ratio (X1) is computed based on aNOx discharged quantity (Y1) from the engine and a NOx quantity (Y2)downstream of the SCR catalyst 42. The NOx discharge quantity (Y1) iscomputed by used of maps or formulas according to the current enginedriving condition (engine speed, fuel injection quantity). The NOxquantity (Y2) is computed based on the output of the NOx sensor 47.

The ECU 50 sends an open valve command signal to the UWA valve 44periodically so that a driving portion (solenoid portion) of the UWAvalve 44 is energized. When the solenoid is energized to open the UWAvalve 44, the urea water is injected from the UWA valve 44. An outputcycle (output frequency) of the open valve command signal is adjusted sothat the urea water adding quantity is increased or decreased. FIG. 2Ashows a base open valve command signal to the UWA valve 44. When theoutput interval of the open valve command signal is made longer thanthat of the base open valve command signal, as shown in FIG. 2B, theurea water adding quantity is decreased. When the output interval of theopen valve command signal is made shorter as shown in FIG. 2C, the ureawater adding quantity is increased. The UWA valve 44 may be temporarilyclosed in order to decrease the urea water adding quantity.

Referring to FIG. 3, a configuration of the NOx sensor 47 will bedescribed, hereinafter. FIG. 3 is a cross sectional view showing asensor element 60 of the NOx sensor 47. The sensor element 60 includes apump cell, sensor cell, and a monitor cell which are laminated. Sincethe monitor cell has a function of discharging Oxygen in the gas as wellas the pump cell, the monitor cell can be referred to as an auxiliarypump cell or a second pump cell.

In the sensor element 60, solid electrolyte layers 61, 62, which aremade of oxygen ion conductive material such as zirconia, are laminatedthrough a spacer 63 which is made of insulating material such asalumina. The upper solid electrolyte layer 61 is provided with anexhaust gas inlet 61 a through which the exhaust gas is introduced intoa first chamber 64. The first chamber 64 communicates with a secondchamber 66 through a restricting portion 65. A porous diffusion layer 67is arranged on an upper surface of the upper solid electrolyte layer 61in order to introduce or discharge the exhaust gas with a specifieddiffusion resistance, and an insulating layer 69 is also arranged on theupper surface of the upper solid electrolyte layer 61 to define anatmosphere passage 69.

An insulating layer 71 is arranged on a lower surface of the lower solidelectrolyte layer 62 to define an atmosphere passage 72.

The lower solid electrolyte layer 62 is provided with a pump cell 81which faces the first chamber 64. The pump cell 81 introduces ordischarges the oxygen into or from the first chamber 64 to adjust aresidual oxygen concentration in the first chamber 64 to a specifiedvalue. The pump cell 81 is provided with a pair of electrodes 82, 83 onits upper surface and its lower surface. The upper electrode 82 in thefirst chamber 64 is a NOx inactive electrode. When a specified voltageis applied between the electrodes 82, 83, the pump cell 81 degradesoxygen in the first chamber 64, so that the degraded oxygen isdischarged into the atmosphere passage 72 from the lower electrode 83.

The upper solid electrolyte layer 61 is provided with a monitor cell 84and a sensor cell 85 which face the second chamber 66. After thedegraded oxygen is discharged by the pump cell 81, the monitor cell 84generates electric power according to the residual oxygen concentrationor generates electric output according to applied electric voltage. Thesensor cell 85 detects NOx concentration of gas in the second chamber66.

The monitor cell 84 and the sensor cell 85 is adjacently aligned, andinclude a pair of electrodes 86, 87 in the second chamber 66 and acommon electrode 88 in the atmosphere passage 88. That is, the monitorcell is comprised of the upper solid electrolyte layer 61, the electrode86 and the common electrode 88, and the sensor cell 85 is comprised ofthe upper solid electrolyte layer 61, the electrode 87 and the commonelectrode 88. The electrode 86 of the monitor cell 84 is made of noblemetal such as Au—Pt, which is inactive to NOx. The electrode 87 of thesensor cell 85 is made of noble metal such as Pt, Rh, which is active toNOx. Although FIG. 3 shows that the monitor cell 84 and the sensor cell85 are aligned in series in an exhaust gas flow, the monitor cell 84 andthe sensor cell 85 are actually arranged in parallel.

A heater 73 is embedded in the insulating layer 71 for heating whole ofthe sensor element 60. The heater 73 receives electricity from a batteryand generates heat energy in order to activate whole of the sensorelement including the pump cell 81, the monitor cell 84 and the sensorcell 85.

In the sensor element 60 described above, the exhaust gas is introducedinto the first chamber 64 through the porous diffusion layer 67 and theexhaust gas inlet 61 a. When the exhaust gas flows around the pump cell81 and a pump-cell voltage is applied between the pump cell electrodes82, 83, the degradation of oxygen occurs so that the oxygen isintroduced or discharged through the pump cell 81 according to theoxygen concentration in the first chamber 64. Since the electrode 82 inthe first chamber 64 is inactive to NOx, NOx is not degraded in the pumpcell 81, but only oxygen is degraded in the pump cell 81 to bedischarged into the atmosphere passage 72 from the electrode 83. Hence,the pump cell 81 maintains the interior of first chamber 64 at aspecified low oxygen concentration.

The gas passed through the pump cell 81 flows into the second chamber66, and the monitor cell 84 generates outputs according to the residualoxygen concentration in the gas. The output of the monitor cell 84 isdetected as a monitor cell current by applying a specified monitor cellvoltage between the monitor cell electrodes 86, 88. Besides, when aspecified sensor cell voltage is applied between the sensor cellelectrodes 87, 88, NOx is reduced and the oxygen is generated. Thegenerated oxygen is discharged into the atmosphere passage 68 from theelectrode 88. At this moment, NOx concentration in the exhaust gas isdetected based on the electric current flowing through the sensor cell85. This electric current is referred to as sensor cell current.

When the urea water is injected by the UWA valve 44, the NOx purifyingratio of the SCR catalyst 42 is computed based on the output of the NOxsensor 47. In such a case, since the NOx sensor senses ammonia (NH₃) aswell as NOx in the exhaust gas, if the excessive ammonia existsdownstream of the SCR catalyst 42 due to the ammonia slip, the NOxsensor erroneously outputs its detected signal due to the ammoniadetection. Hence, the NOx purifying ratio is erroneously computed, sothat the urea water adding quantity can not be precisely controlled.

That is, when ammonia is excessive in the SCR catalyst 42 and theexcessive ammonia is discharged downstream of the SCR catalyst 42, thegas containing ammonia flows from the first chamber 64 to the secondchamber 66 and the chemical reaction of ammonia is arisen in the sensorcell 85. Specifically, an oxidation reaction is arisen in the sensorcell 85 as shown in following chemical equation, and the output of theNOx sensor increases along with the oxidation reaction.

4NH₃+5O₂→4NO+6H₂O  (5)

FIGS. 4A to 4C respectively show relationship between the urea wateradding quantity and the NOx concentration downstream of the catalyst,between the urea water adding quantity and the NH₃ concentration, andbetween the urea water adding quantity and the NOx sensor output.

As shown in FIGS. 4A and 4B, when the urea water adding quantity isincreased, NOx is purified by the SCR catalyst 42 to be decreased andNH₃ concentration is increased around after the NOx purifying ratio issaturated. As described above, since the NOx sensor senses ammonia aswell as NOx, the output of the NOx sensor increases as the NOx and theammonia are increased. That is, the NOx sensor output line is downwardlyconvex with respect to the urea water adding quantity as shown in FIG.4C.

In FIG. 4C, when the urea water adding quantity is “A1”, the NOx sensoroutput is a minimum value. In a region where the urea water addingquantity is less than “A1”, as the urea adding water quantity increases,the NOx purifying ratio increases and the NOx sensor output decreases.In a region where the urea water adding quantity is greater than “A1”,as the urea adding water quantity decreases, the NOx sensor outputdecreases. Hence, the urea water adding quantity in which the NOx sensoroutput is minimum corresponds to the water adding quantity in which theammonia slip quantity is small and the NOx purifying ratio is maximum.By controlling the urea water adding quantity in such a manner that theNOx sensor output becomes minimum, the NOx purifying ratio becomes highand the ammonia slip quantity becomes minimum.

In the characteristic shown in FIG. 4C, a variation gradient of the NOxsensor output relative to the variation in the urea water addingquantity is small around the minimum value of the NOx sensor output. Asthe urea water adding quantity is apart from the minimum value, thevariation gradient of the NOx sensor output becomes large. Some NOxsensors have no variation gradient of its output around the minimumvalue thereof.

According to the present embodiment, the urea water adding quantity isadjusted and the NOx sensor output is obtained every urea water addingquantity. An adding quantity command value is computed based on the ureawater adding quantity in which the NOx sensor output is minimum. Thecomputed adding quantity command value is established as a target value,and the urea water adding quantity is controlled so as to agree with thetarget value.

Referring to FIG. 5, a urea water adding control process will bedescribed hereinafter, This process is repeatedly performed by the ECU50 at a predetermined time interval, In step S101, the computerdetermines whether the SCR catalyst 42 is activated. Specifically, thecomputer determines whether the exhaust gas temperature is greater thana specified value (for example, 150° C.). When the answer is Yes, theprocedure proceeds to step S102.

In steps S102-S104, the computer determines whether an executioncondition for the urea water adding quantity control is established.Specifically, in step S102, the computer determines whether an enginespeed variation ΔNE between a previous engine speed and a current valuespeed is less than a specified value ΔNE0. In step S103 the computerdetermines whether a fuel injection quantity variation ΔQ between aprevious fuel injection quantity and a current fuel injection quantityis less than a specified value ΔQ0. In step S104, the computerdetermines whether the NOx purifying ratio is less than a specifiedratio “R0”. That is, in steps S102 and S103, the computer determineswhether the engine driving condition is stable. In step S104, thecomputer determines whether it is necessary to update the addingquantity command value based on the current NOx purifying ratio.

When any of answers in steps S102-S104 is NO, the procedure proceeds tostep S105. When all answers in steps S102-S104 are YES, the procedureproceeds to step S106. In step S105, the urea water addition isperformed based on the current adding quantity command value. Thiscorresponds to a normal urea water adding process.

In step S106, an increase/decrease process of the urea water addingquantity is performed in order to update the adding quantity commandvalue. Referring to FIG. 6, the increase/decrease process of the ureawater adding quantity will be described in detail. In theincrease/decrease process, the urea water adding quantity is varied inthe order of the adding quantity command value, an adding quantityincrease value, and an adding quantity decrease value. The NOx sensoroutput is successively obtained. A period where the urea water additionis performed based on the adding quantity command value is referred toas a first period, a period where the urea water addition is performedbased on the adding quantity increase value is referred to as a secondperiod, and a period where the urea water addition is performed based onthe adding quantity decrease value is referred to as a third period,hereinafter.

In step S201, the computer determines whether it is in the first periodnow. When the answer is YES, the procedure proceeds to step S202 inwhich the urea water addition is performed based on the adding quantitycommand value. And then, the procedure proceeds to step S203 in which aNOx sensor output V1 is stored.

When the answer is NO in step S201, the procedure proceeds to step S204in which the computer determines whether it is in the second period.When the answer is YES in step S204, the procedure proceeds to step S205in which the adding quantity command value is increased by a specifiedvalue α and the urea water addition is performed based on the addingquantity increase value (adding quantity command value+α). Then, theprocedure proceeds to step S206 in which a NOx sensor output V2 isstored.

When the answer is NO in step S204, it can be assumed that it is in thethird period. The procedure proceeds to step S207 in which the addingquantity command value is decreased by the specified value α and theurea water addition is performed based on the adding quantity decreasevalue (adding quantity command value−α). Then, the procedure proceeds tostep S208 in which a NOx sensor output V3 is stored.

Referring back to FIG. 5, in step S107, the computer determines whetherthe above increase/decrease process has been completed. When theincrease/decrease process shown in FIG. 6 has been completed, theprocedure proceeds to step S108. When the answer is NO in step S107, theprocedure ends once.

In step S108, the computer derives a minimum sensor output Vmin from theabove NOx sensor outputs V1-V3. In step S109, the computer derives amaximum sensor output Vmax from the above NOx sensor outputs V1-V3.

Then, the procedure proceeds to step S110 in which the computerdetermines whether a difference between the maximum sensor output Vmaxand the minimum sensor output Vmin is greater than a specified value V0.When the answer is NO in step S110, the computer determines that it isunnecessary to update the adding quantity command value, and theprocedure ends. When the answer is YES in step S110, the procedureproceeds to step S11.

In step S111, the computer determines whether the NOx sensor output V1is minimum sensor output Vmin, When the answer is NO, the procedureproceeds to step S112. When the answer is YES, the procedure proceeds tostep S113.

In step S112, the urea water adding quantity corresponding to theminimum sensor output Vmin is stored in the EEPROM 51 as the addingquantity command value. At this moment, one of the adding quantityincrease value or the adding quantity decrease value is stored as a newadding quantity command value. This new adding quantity command valuecorresponds to a learning value. Thereby, the update of the addingquantity command value is completed.

In step S113, the variation width to the adding quantity command valueis made small, and the adding quantity increase/decrease process isperformed again. After the adding quantity increase/decrease process isperformed again, the adding quantity command value is updated (stepS108-S12).

Alternatively, when the answer is repeatedly YES for specified times instep S111, the variation width is made small and the adding quantityincrease/decrease process is performed again. That is, when the ureawater quantity increase/decrease process and the minimum sensor outputretrieval have been repeatedly performed, and when the NOx sensor outputbased on the original adding quantity command value is the minimumsensor output Vmin repeatedly for the specified times, theincrease/decrease process (the process shown in FIG. 6) is performedagain.

The adding quantity command value in which the NOx sensor output isminimum does not vary successively. When the engine driving condition isstable, the adding quantity command value is constant value. Thus, it isunnecessary to compute the adding quantity command value repeatedly whenthe engine driving condition is stable. For example, every when the ECUis energized, the adding quantity command value may be computed onlyonce.

The characteristic of NOx sensor output with respect to the urea wateradding quantity varies according to the engine driving condition. Morespecifically, when the exhaust gas quantity or the exhaust gastemperature is varied according to the engine driving condition, thecharacteristic of the NOx sensor output is varied. As shown in FIG. 7,when the exhaust gas quantity is increased, the characteristic of thesensor output varies from “L1” to “L2”. Also when the exhaust gastemperature is decreased, the characteristic varies in the same manner.Thus, it is desirable that the adding quantity command value is learnedwith the engine driving condition. For example, the engine load(accelerator operation amount) and the engine speed are established asdriving condition parameters, and the adding quantity command value islearned with respect to each parameter.

Alternatively, the adding quantity command value can be learned withrespect to the exhaust gas quantity and the exhaust gas temperature.

The process of the adding quantity increase/decrease process and theminimum sensor output retrieval will be described more specifically,hereinafter. FIGS. 8A and 8B are time charts showing a transition of theNOx sensor output when the urea water adding quantity is varied. FIGS.9A-10B are graphs showing the characteristic of the NOx sensor outputschematically.

FIGS. 8A and 8B show the first period T1, the second period T2, and thethird period T3. FIGS. 9A-10B correspond to FIG. 4C. In FIGS. 9A-10B,the characteristics of the NOx sensor output is illustrated as V-shapein order to easily explain the NOx sensor output. In FIGS. 9A-10B, Romannumbers I, II, III represent a variation order of the urea water addingquantity. The Roman number “I” represents the urea water addition by theadding quantity command value, the number “II” represents the urea wateraddition by the adding quantity increase value, and the number “III”represents the urea water addition by the adding quantity decreasevalue.

The NOx sensor output varies according to the urea water addingquantity.

For example, in a case shown in FIG. 9A, the adding quantity commandvalue, the adding quantity increase value, and the adding quantitydecrease value are greater than the urea water adding quantitycorresponding to the minimum sensor output. The NOx sensor output V3 isa minimum value. Thus, the adding quantity command value is updated bythe urea water adding quantity corresponding to the NOx sensor outputV3.

FIG. 8A is a time chart showing the case of FIG. 9A. As shown in FIG.8A, an output interval of an open valve command pulse of the UWA valve44 is varied so that the urea water adding quantity is varied. Alongwith the variation in the urea water adding quantity, the NOx sensoroutputs V1, V2, V3 are obtained and the NOx sensor output V3 is theminimum sensor output Vmin.

The third period T3 is longer than the second period T2. Comparing acase that the urea water adding quantity is increased with a case thatthe urea water adding quantity is decreased, a response speed of the NOxsensor differs. The response speed is slow in the latter case. This isbecause ammonia consuming speed in the SCR catalyst 42 is slower thanammonia adsorbing speed in the SCR catalyst 42. As described above, bymaking difference in the urea water adding period between a case wherethe urea water quantity is increased and a case where the urea waterquantity is decreased, the sensor output is appropriately obtained evenif the urea water adding quantity is increased or decreased. Besides, inboth cases, a minimum required time period can be established.

After the NOx sensor output V3 is obtained as the minimum sensor outputVmin, a similar adding quantity increase/decrease process issuccessively performed to retrieve a minimum sensor output Vmin. In thiscase, as shown in FIGS. 8B and 9B, the urea water addition is performedby the previously computed adding quantity command value (V3 in FIG. 9A)as a current adding quantity command value. After the NOx sensor outputV1′ is obtained, the urea water addition by the adding quantity increasevalue and the urea water addition by the adding quantity decrease valueare performed. Then, the NOx sensor outputs V2′ and V3′ are obtained.The NOx sensor output V1′ is a minimum sensor output, and the addingquantity command value is not updated. Besides, in FIG. 8B, a value of(V2′−V1′) corresponds to a variation width (Vmax−Vmin). If thisvariation width is less than a specified value, the adding quantitycommand value is not updated.

When the adding quantity increase/decrease process shown in FIGS. 9A and9Bn is successively performed, it is previously known that the NOxsensor output is increased (V2′>V1′) in a case of increasing the addingquantity shown in FIG. 9B. Hence, the urea water adding quantity can beonly decreased.

FIGS. 10A and 10B show a case where the adding quantityincrease/decrease process is performed with respect to the currentadding quantity command value and then the adding quantityincrease/decrease process is performed again while the variation widthis made small.

As shown in FIG. 10A, with respect to the adding quantity command value,the adding quantity increase value, and the adding quantity decreasevalue, the NOx sensor outputs V1-V3 are obtained respectively. In thiscase, the NOx sensor output V1 is the minimum sensor output Vmin. Thereis a possibility that a more appropriate adding quantity command valuemay exists between the maximum adding quantity (adding quantity increasevalue) and the minimum adding quantity (adding quantity decrease value),in which the NOx sensor output becomes smaller. Thus, as shown in FIG.10B, the adding quantity increase/decrease process is performed againwhile the variation width relative to the adding quantity command valueis made smaller.

In FIG. 10B, since the NOx sensor output V3′ is the minimum sensoroutput, the adding quantity command value is updated by the urea wateradding quantity corresponding to the NOx sensor output V3′.

According to the present embodiment, following advantages can beobtained.

The urea water addition is performed while the urea water addingquantity is varied, and the NOx sensor output is obtained with respectto each adding quantity. The adding quantity command value is computedbased on the urea water adding quantity in which the sensor output isthe minimum value. Hence, the urea water adding quantity in which theammonia slip quantity is small and the NOx purifying ratio is maximumcan be established as the adding quantity command value. As the result,the NOx quantity downstream of the SCR catalyst 42 is correctlydetected, so that the NOx purifying ratio can be properly computed.

Further, while the engine is running, the urea water addition by theurea water adding valve 44 is performed with the adding quantity commandvalue as the target value, so that the NOx purifying ratio can bemaintained at a maximum value. A discharge of ammonia to the downstreamside of the SCR catalyst 42 is restricted as much as possible. That is,the NOx purifying ratio can be kept high and the ammonia slip can bereduced.

Three steps of the urea water addition are performed while the addingquantity command value is increased/decreased by a specified variationwidth. When the middle quantity of the three step urea water additioncorresponds to the minimum NOx sensor output, the variation width ismade smaller and the urea water adding quantity is varied again. Thus,the most appropriate adding quantity command value can be correctlyobtained.

Only when the NOx purifying ratio is relatively low, the adding quantitycommand value is updated. Hence, unnecessary computation (update) of theadding quantity command value can be avoided when the NOx purifyingratio is appropriate and the adding quantity command value does not needto be updated.

When the difference between the maximum sensor output Vmax and theminimum sensor output Vmin is less than the specified value V0, theadding quantity command value is not newly computed. Thus, unnecessarycomputation (update) of the adding quantity command value can beavoided.

Since the adding quantity command value in which the NOx sensor outputis minimum is stored in the EEPROM 51 as the learning value, thecharacteristic of the NOx sensor is constantly obtained to preferablyperform the urea water addition. Besides, since the adding quantitycommand value may be computed in a minimum frequency, a computation loadfor computing the adding quantity command value can be reduced.

Further, since the adding quantity command value is stored in the EEPROM51 with the engine driving condition, an appropriate adding quantitycommand value can be established even if the engine driving condition ischanged.

The present invention is not limited to the embodiments described above,but may be performed, for example, in the following manner.

-   -   In the above embodiment, while the variation width is constant        in varying the urea water adding quantity, the urea water adding        quantity is varied from the adding quantity increase value to        the adding quantity decrease value across the adding quantity        command value. This order can be changed as follows. For        example, as shown in FIG. 11A, the urea water adding quantity        can be varied in an increasing direction only. In FIG. 11A, the        urea water adding quantity is increased by a constant quantity        in the order of the Roman numbers I to IV and the NOx sensor        output is obtained. Alternatively, as shown in FIG. 11B, the        urea water adding quantity may be increased while the variation        width is being changed. In FIG. 11B, the variation quantity is        relatively large when the urea water adding quantity is varied        from “I” to “III” through “II”. Then, the variation width is        made small when the urea water adding quantity is varied from        “III” to “V” through “IV”.    -   The variation width of the urea water adding amount may be set        according to the NOx sensor output. For example, as the NOx        sensor output increases, the variation width of the urea water        adding quantity increases. As shown in FIG. 4, when the NOx        sensor output is relatively large, the variation ratio of the        NOx sensor output is also relatively large. When the NOx sensor        output is relatively small, the variation ratio of the NOx        sensor output is also relatively small. Hence, by setting the        variation width of the urea water adding quantity based on the        NOx sensor output, the NOx sensor minimum output can be        appropriately obtained.    -   Based on a difference between the NOx sensor output before        changing the urea water adding quantity and the NOx sensor        output after changing the urea water adding quantity, it can be        estimated whether the urea water adding quantity in which the        NOx sensor output is minimum is in an increasing side or a        decreasing side. Based on this estimated result, the urea water        adding quantity may be increased or decreased. For example, when        the NOx sensor output is increased due to the variation in the        urea water adding quantity, an increase/decrease direction of        the urea water adding quantity is reversed. Alternatively, when        the NOx sensor output is decreased due to the variation in the        urea water adding quantity, the urea water adding quantity is        varied in the same increase/decrease direction. According to        this configuration, the urea water adding quantity is varied in        a direction where the minimum sensor output exists. Thus, the        increase/decrease process of the urea water adding quantity can        be simplified.    -   When it is determined that the ammonia slip catalyst 43 is        inactive, the increase/decrease operation of the urea water        adding quantity and the update of the urea water adding command        value may be performed. Specifically, the ECU 50 determines the        condition of the ammonia slip catalyst 43 based on the        temperature of the catalyst 43 or an elapsed time after the        engine is started. When it is determined that the ammonia slip        catalyst 43 is inactive, the increase/decrease process of the        urea water adding quantity is performed. In such a case, the        discharge of ammonia downstream of the SCR catalyst 42 is        restricted and a discharge of ammonia to the atmosphere is        restricted.    -   In the above embodiment, the urea water adding quantity is        controlled by adjusting the output interval of the open valve        command pulse to the UWA valve 44. Alternatively, the urea water        adding quantity can be controlled by adjusting a pulse length of        the open valve command pulse.    -   The NOx sensor can employ a configuration other than the        configuration shown in FIG. 3. For example, the NOx sensor may        include a pump cell and a sensor cell but a monitor cell (second        pump cell). Alternatively, the oxygen pumping may be performed        between the pump cell and the atmosphere.    -   As a reducing agent adding means, a urea water adding nozzle can        be used in stead of the UWA valve 44.

1. An exhaust gas purifying apparatus for an internal combustion enginewhich includes a NOx reduction catalyst provided in an exhaust gas pipe,a reducing agent adding means for adding a reducing agent upstream ofthe NOx reduction catalyst, and a NOx sensor detecting a NOx quantitydownstream of the NOx reduction catalyst, the exhaust gas purifyingapparatus performing a reducing agent addition by means of the reducingagent adding means according to an adding quantity command value of atarget value, the exhaust gas purifying apparatus comprising; an addingquantity control means for controlling an adding quantity of thereducing agent added by the reducing agent adding means; and a commandvalue computing means for computing the adding quantity command value inwhich a sensor output of the NOx sensor becomes minimum while obtainingthe sensor output with respect to every adding quantity of the reducingagent controlled by the adding quantity control means.
 2. An exhaust gaspurifying apparatus according to claim 1, wherein the adding quantitycontrol means varies an adding quantity of the reducing agent at leastinto increasing side or decreasing side relative to the adding quantitycommand value as a reference.
 3. An exhaust gas purifying apparatusaccording to claim 1, wherein the adding quantity control means performsat least three steps of reducing agent additions while the addingquantity of the reducing agent is varied by a predetermined variationwidth, and the adding quantity control means performs the reducing agentaddition again while the variation width is made small in a case thatthe sensor output becomes minimum with respect to a medium quantity ofreducing agent among at least three steps of reducing agent additions 4.An exhaust gas purifying apparatus according to claim 1, furthercomprising an estimation means for estimating whether an adding quantityof the reducing agent for obtaining minimum value of the sensor outputis in a decreasing side or increasing side based on the sensor outputbefore varying the adding quantity and the sensor output after varyingthe adding quantity, wherein the adding quantity control means increasesor decreases the adding quantity of the reducing agent based on anestimation by the estimation means.
 5. An exhaust gas purifyingapparatus according to claim 1, wherein the adding quantity controlmeans sets a variation width of the adding quantity of the reducingagent based on the output value of the NOx sensor.
 6. An exhaust gaspurifying apparatus according to claim 1, wherein the command valuecomputing means does not newly compute the adding quantity command valuewhen a difference between a minimum value of the sensor output and amaximum value of the sensor output, which are obtained due to avariation in adding quantity of the reducing agent is within a specifiedvalue.
 7. An exhaust gas purifying apparatus according to claim 1,wherein the adding quantity control means decreases the adding quantityin a specified injection interval which is longer than another injectioninterval in which the adding quantity is increased.
 8. An exhaust gaspurifying apparatus according to claim 1, further comprising a learningmeans for storing the adding quantity command value in a back-up memoryas a learning value and for updating the learning value as needed.
 9. Anexhaust gas purifying apparatus according to claim 8, wherein thelearning means stores the adding quantity command value along with adriving condition of the internal combustion engine at a time ofcontrolling the adding quantity of the reducing agent.
 10. An exhaustgas purifying apparatus according to claim 1, further comprising: anoxidation catalyst arranged downstream of the NOx reduction catalyst forpurifying the reducing agent; and a determination means for determiningwhether the oxidation catalyst is active or not, wherein when thedetermination means determines the oxidation catalyst is inactive, thecommand value computing means performs a computation of the addingquantity command value.
 11. An exhaust gas purifying apparatus accordingto claim 1, wherein the NOx sensor is provided with a sensor elementwhich includes a solid electrolyte and a NOx detecting electrodeprovided on the solid electrolyte.